An IGBT comprises an emitter region of a first conductive type, a drift region of the first conductive type, and a body region of a second conductive type separating the emitter region and the drift region. A technique is proposed for reducing on-voltage of the IGBT by providing a semiconductor region of the first conductive type within the body region of the second conductive type. This technique is disclosed in: Proc. of the 6th internat. Symposium on Power Semiconductor Devices & IC's, Davos, Switzerland. 1994. “Trench Gate Emitter Switched Thyristors” M. S. Shekar, J. Korec, B. J. Baliga. p 189-194. IEEE Cat. no. 94CH3377-9.
FIG. 16 (a) schematically shows a cross-sectional view of essential parts of an IGBT 100 disclosed in the above document. The IGBT 100 comprises n+ type emitter regions 132, a p− type body region 128 making contact with the emitter regions 132, and an n− type drift region 126 that makes contact with the body region 128 and is isolated from the emitter regions 132 by the body region 128. The IGBT 100 further comprises trench gate electrodes 142. The trench gate electrodes 142 pass through the body region 128 at portions separating the emitter regions 132 and the drift region 126, and extend from the emitter regions 132 to the drift region 126. The trench gate electrode 142 faces the body region 128 via a gate insulator film 144. The IGBT 100 further comprises an n+ type semiconductor region 154 formed within the body region 128. The semiconductor region 154 is isolated from the emitter regions 132 by a body region 128a, and is also isolated from the drift region 126 by a body region 128b. The body region 128a and the body region 128b are connected to each other at a cross-sectional plane (not shown).
A p+ type body contact region 134 is formed in a region between left and right emitter regions 132 at an upper part of the body region 128a. An n+ type buffer region 124 and a p+ type collector region 122 are formed in sequence below the drift region 126. The emitter regions 132 and the body contact region 134 are electrically connected to an emitter electrode E. The collector region 122 is electrically connected to a collector electrode C.
The IGBT 100 is characterized in comprising the semiconductor region 154 within the body region 128. The semiconductor region 154 extends from the gate insulator film 144 of one of the trench gate electrodes 142 to the gate insulator film 144 of the other of the trench gate electrodes 142. The semiconductor region 154 is also insulated from the emitter regions 132, the body regions 128a and 128b, and the drift region 126. The potential of the semiconductor region 154 is not determined directly by the potential applied to the IGBT 100, but fluctuates according to surrounding environment. This potential state is referred to as floating in the present specification.
When the emitter electrode E of the IGBT 100 is earthed, a positive voltage is applied to the collector electrode C, and a positive gate voltage is applied to the trench gate electrodes 142, portions of the body region 128 facing the trench gate electrodes 142 are inverted to the n type, and channel regions are formed. Electron carriers are supplied from the emitter regions 132, are injected into the drift region 126 via the channel regions that were inverted to the n type, and accumulate in the buffer region 124. When the electron carriers accumulate in the buffer region 124, there is a reduction of the contact potential difference of the buffer region 124 and the collector region 122. As a result, positive hole carriers are injected from the collector region 122 toward the buffer region 124 and the drift region 126, and electric conductivity modulation occurs.
The positive hole carriers that have been injected from the collector region 122 recombine with the electron carriers and disappear, or are emitted from the emitter electrode E via the body region 128 and the body contact region 134.
When the semiconductor region 154 is provided within the body region 128, a potential barrier is formed between the semiconductor region 154 and the body region 128b. This potential barrier obstructs the flow of the positive hole carriers that have been injected from the collector region 122. The positive hole carriers thus accumulate in the body region 128b and the drift region 126, and the on-voltage of the IGBT 100 decreases.